Wood strand board

ABSTRACT

A method of production of and a mineral bonded wood strand cement (magnesite, gypsum) board which is produced from a mixture of wood strands or similar fiber material, water, a suitable binder like cement, one or both of magnesite and gypsum, and possibly additives, which mixture is cured under pressure to form a board with a practically entirely closed surface. For the boards according to the invention, wood strands are being used that are broader and wider than used until presently in acoustic wood wool cement (magnesite, gypsum) boards.

The invention relates to a mineral bonded Wood Strand Cement (Magnesite, Gypsum) Board which is composed of a mixture of wood strands or other similar fibre material (such as wood wool, wafers, chips, flakes, et cetera), a suitable binder such as cement, magnesite or gypsum and possibly additives, which mixture is cured under pressure to form a board with a practically fully closed surface.

Wood Cement Boards and Fibre Cement Boards are used worldwide in a large number of varying applications. Advantages of these boards are, amongst others, the durability and the resistance against fire, moisture, rot, fungi and termites.

Wood Wool Cement Boards have been known for almost a century. These boards are made of wood wool, preferably curled fibres of restricted width, and cement, magnesite or gypsum. These known boards have an open matrix and therefore a relatively low density. Initially these boards were used for insulation purposes and as a base for stuccoed ceilings. Nowadays these boards are also used for decoration purposes and as sound absorbing boards. These applications are made possible by the introduction of automated machines for the production of these boards. With the development of machines with control systems that control the accurate dosing and mixing of the raw materials, the even distribution of the mixture in the moulds and the stacking and pressing of the moulds with the fresh mixture, the production of boards of a constant quality is feasible. With the availability of these machines it is possible to produce boards of high quality at low costs. In particular by using wooden moulds (concrete form boards) and a low pressure press, the costs for the equipment of a production line of Wood Wool Cement Boards are relatively low.

However, because of their open matrix structure, Wood Wool Cement Boards are unsuitable for structural applications like external wall claddings and roofing, for which a closed and water proof surface is important. Moreover, the strength of the Wood Wool Cement Boards is not sufficient for such applications.

To the present day applications like external claddings and roofing are amongst others made with corrugated or flat Fibre Cement Boards or shingles. These boards and products have a closed matrix and are waterproof. Optionally, flat boards can be provided with a relief in the surface, for example with a texture of slate, brick or imitation wood grain. Until recently these Fibre Cement Boards were mainly made from cement and asbestos fibres, which were suitable because of their durability, strength and low costs. In many countries asbestos fibres are now replaced by cellulose and synthetic fibres, mainly due to the legislation which forbids the use of asbestos because of the danger for the manufacturers and the processors of the boards. A disadvantage of the fibre cement boards is, however, that these boards are not as strong and not as durable as the asbestos boards and moreover synthetic fibres and strong cellulose are expensive.

The Dutch patent 1 0010 195 describes Wood Strand Cement Boards which possess the advantages of Wood and Fibre Cement Boards concerning the durability and the resistance against fire, moisture, rot, fungi and termites.

However, these boards have a number of essential disadvantages.

One of the disadvantages is that in the compressed upper layer made of cement mortar or an other sealing material, hair cracks can appear in the surface specially in case the cement layer is not supplied with a coating finish such as paint. An other disadvantage is that due to the relatively low density of these boards, of 800 to 900 kg per m³, the bending strength leaves much to be desired and further the screwability and nailability and screw and nail holding capability of these boards is problematic.

Other problems arise in the processing of such boards, like profiling, applying tongues and grooves and beveling of the edges of the boards. Further these boards may curve under the influence of a varying moisture content due to being not balanced with identical surfaces.

The invention under consideration aims to provide a Wood Strand Cement (Magnesite, Gypsum) Board that can annihilate the aforementioned disadvantages.

To this aim the present invention provides a Wood Strand Cement (Magnesite, Gypsum) Board that is made from a mixture of wood strands or a similar fibre material, water, a suitable binder like cement, magnesite or gypsum and possible supplements, which mixture is cured under pressure to form a board with a practically fully closed surface, characterised in that the wood strands are broader but thinner than the wood wool that is used until today in Wood Wool Cement (Magnesite, Gypsum) Boards for acoustic absorption purposes.

Surprisingly it has been shown that with the use of broader and thinner wood strands compared to the wood strands as used in the acoustic Wood Wool Cement Boards up to date and by omitting the compressed upper layer composed of cement mortar or other sealing material, the aforementioned disadvantages are nullified quite effectively.

In order to achieve better results, it is advantageous to have a width of the wood strands of 1-12 mm or more, preferably 3-10 mm, and a thickness of 0.15-1.0 mm, preferably 0.15-0.3 mm.

Surprisingly it has been shown that with the use of wood strands with sharp edges, the edges of the wood strands do no longer show. Furthermore it shows that whenever the wood strands are distributed at random, a smooth and almost ridge free surface is obtained after pressing without the use of a coating layer of saw dust and cement as described in the Dutch patent 1 010 195.

Since the boards according to the invention can have a density of between 900 and 1200 kg per m³, preferably of 1000-1100 kg per m³, it has turned out that the boards have a higher bending strength compared to the known boards as described in the before mentioned Dutch patent, which have a density of 800 to 900 kg per m³.

Moreover, the boards according to the invention are better nailable and screwable and the screws, staples and nails hold better in the new material.

This higher density of the boards under consideration turns out to result in a considerable improvement in relation to the processing of the boards, such as profiling, the application of grooves and beveling of the edges of the boards.

It is noted that although the board can be painted or stuccoed, there is no need for stuccoing on account of the particularly smooth surface. It has been proven that without further surface treatment like painting, the boards are weatherproof.

Surprisingly it has also turned out that the use of long wood strands according to the invention leads to a bending strength of more than double the Norm set for Cement Bonded Particle Boards at an approximately 20% lower density and in addition makes the boards considerably cheaper to produce. The boards according to the invention can be supplied on one side with an embossing of for instance a brick, slate or wood grain texture, which makes them suitable as durable cladding boards or planks on the exterior of a building.

An other important advantage of the boards under consideration is their resistance to fire, moisture, rot and fungi, as well as thaw and frost. In addition, they are resistant to all weather conditions and the devastating effect of termites.

The boards according to the invention are very durable and can be applied successfully in tropical countries that are afflicted by hurricanes, rainstorms and earthquakes like in Central America (Honduras, Guatemala, El Salvador, Nicaragua).

Furthermore, the invention relates to the operating process for the production of Wood Strand Cement Boards, which is characterized by the following steps:

-   -   a) distribution into moulds of a mixture consisting of wood         strands, water, and cement, magnesite or gypsum;     -   b) stacking of the moulds with raw material up to the desired         height;     -   c) pressing of the stack of moulds with raw material, in order         to achieve the desired density of the boards; and     -   d) maintaining of this height through a connection of a         so-called Bottom which lies beneath the stack, with a Top, which         lies on top of the stack, using tie rods during the initial         stage of curing of the cement, magnesite or gypsum.

Wood strands with a width of 1-12 mm or more, preferably 3-10 mm, and a thickness of 0.15-1.0 mm, preferably 0.15-0.3 mm are used.

It is particularly advantageous to use wood strands with pointed edges, especially if angles of less than 120° are provided on the strands.

Besides, for the production of the boards both moulds of plywood as cauls of steel can be used.

For the production of Wood Strand Cement (Magnesite, Gypsum) Boards with a relief surface according to the invention a separate board with a counter-profile is placed in the mould respectively the top of the mould itself and is provided with a counter-profile.

For certain reliefs it is advantageous when onto the counter-relief board a mixture of wood strands, water and cement is distributed, which subsequently is being pressed to achieve the wanted relief, without using an additional sealing layer of cement, magnesite or gypsum possibly mixed with short fibres such as saw dust.

In a preferred implementation of the operating procedure according to the invention, at least part of the wood strands is oriented more longitudinally than transversely to the length of the board during the application and distribution of the mixture of wood strands, water and cement, magnesite or gypsum. Due to such orientation of the wood strands lengthwise in relation to the board, the strength and the elasticity of the board will increase in longitudinal direction and the linear stability in this direction due to variations in moisture content is improved.

In case the boards are made with sufficient thickness, they can be sawed into lath or strips of timber, or planks. I beam and other profiles are assembled from strips, lath or planks obtained from the invention. Such lath can be glued together to I beams. Those laths, planks and beams are, because of their strength and stability and their resistance to fire and termites, very suitable for applications in construction work, in particular in countries with many wooden buildings, such as the United States, Scandinavia, Canada, and so on. Such planks can also be favorably used for durable exterior claddings of buildings.

It is obvious that the invention under consideration is not restricted to the claims. 

1. A wood strand cement board, manufactured from a mixture comprising wood strands or fiber material, a magnesite, gypsum, and water, which mixture is cured to form a board with a practically entirely closed surface, wherein the wood strands are broader and thinner than wood wool fibers used in wood wool cement boards.
 2. A board as described in claim 1, wherein the wood strands have a width of 1 to 12 and a thickness of 0.15-1.0 mm.
 3. A board as described in claim 1, wherein edges of the strands are pointed.
 4. A board as described in claim 3, wherein the edges of the strands are sharp with angles less than 120°.
 5. A method for the production of wood strand cement boards, the method comprising the steps of: a) distributing in moulds a mixture comprising wood strands, water, magnesite or gypsum. b) stacking the moulds with the mixture to the desired height; c) pressing the stack of moulds with the mixture, such that the boards acquire a desired density; d) maintaining the required height through the use of a bottom below the stack and a top on top of the stack, connected by tie rods during the initial curing stage of the magnesite or gypsum.
 6. A method as described in claim 5, wherein the wood strands have a width of 1 to 12 mm.
 7. A method as described in claim 6, wherein the wood strands have a thickness of 0.15 to 1.0 mm.
 8. A method as described in claim 5, wherein wood strands have pointed edges.
 9. A method as described in claim 8, wherein the edges of the strands are sharp with angles less than 120°.
 10. A method as described in claim 5, wherein the bottom is situated with a counter-profile, and the top is situated with a counter-profile.
 11. A method as described in claim 5, wherein the mixture is distributed onto a relief board, which subsequently is pressed to yield a profile without additional use of cement, magnetite or gypsum.
 12. A method as described in claim 5, wherein in the distribution step at least part of the wood strands are oriented more in a longitudinal direction than in a transversal direction in relation to a length of the board.
 13. A product from which beams, strips, lath or planks are sawn from relatively thick boards as described in claim
 1. 14. A product from which I beam and other profiles are assembled from strips, lath or planks obtained according to claim
 13. 15. A board as described in claim 2, wherein the wood strands have a width of 3 to 10 mm, and a thickness of 0.15 to 0.3 mm.
 16. A method as described in claim 6, wherein the wood strands have a width of 3 to 10 mm.
 17. A method as described in claim 7, wherein the wood strands have a thickness of 0.15 to 0.3 mm. 